Typhoid fever remains a serious cause of morbidity and mortality throughout most of the world especially in young children. The pathogenesis of typhoid fever is incompletely understood and the role of the major surface antigens, Vi capsular polysaccharide (Vi), lipoopolysaccharide (0 antigen) and flagella antigen (H) in the pathogenesis of the disease and elicitation of human immunity conferring resistance to disease is incompletely understood and controversial. Unfortunately, Salmonella typhosa, the cause of the typhoid fever, is a pathogen only for humans and there are no satisfactory animal models. The purified Vi capsular polysaccharide has been prepard and its protective activity in the mouse potency test (used to standardized host cell in inactivated typhoid vaccines) shows a higher specific activity than our current vaccines and the US standard. The purified Vi polysaccharide has been used as antigen for the diagnosis of asymtomatic carriage in active disease. In this, it has shown a high degree of specificity and accuracy. Its use as a vaccine, both in the purified form, and covalently bound to a T dependent carrier protein is under investigation.